1. Field of the Invention
The present invention is in the field of night vision devices. More particularly, the present invention relates to an image intensifier tube usable in such night vision devices. Such image intensifier tubes are generally responsive to infrared radiation to provide an image in visible light which is replicative of a scene which may be too dim to be viewed with the unaided natural human vision. Still more particularly, the present invention relates to a photocathode for use in such an image intensifier tube, which photocathode according to the preferred embodiment includes integral structure for establishing and maintaining a precise fine-dimension spacing between the photocathode and a microchannel plate of the image intensifier tube. In other words, in the preferred embodiment, part of the photocathode extends to and physically touches the microchannel plate to establish a minimal spacing dimension between the photocathode and the microchannel plate. Further, the present invention relates to a method of making such a photocathode and an image intensifier tube including such a photocathode.
2. Related Technology
Image intensifier tubes which are responsive to low-level visible or infrared light are commonly used in night vision systems. Night vision systems are used by military and law enforcement personnel for conducting operations in low light conditions, or at night. Further, such night vision devices find many civilian uses for hunting, conservation, industrial observations in low-light conditions, and many other uses. For example, night vision systems are used by pilots of helicopters and airplanes to assist their ability to fly at night.
A night vision system converts the available low-intensity ambient light of the visible spectrum, and also at the near infrared portion of the invisible infrared spectrum to a visible image. These systems require some minimal level of ambient light, such as moon light or star light, in which to operate. This minimal level of ambient light may be infrared light which does not provide visibility to the natural human vision. The ambient light is intensified by the night vision system to produce an output image which is visible to the human eye. The present generation of night vision systems utilize image intensification technologies to intensify the low-level visible light as well as the near-infrared invisible light. This image intensification process involves conversation of the received ambient light into electron patterns, intensification of the electron patterns while retaining the relative intensity levels and contrast of the scene, and projection of the electron patterns onto a phosphor screen for conversion into a visible-light image for the operator. The visible-light image is then viewed by an operator of the night vision system through a lens provided in an eyepiece of the system.
The typical night vision system has an optics portion and a control portion. The optics portion comprises lenses for focusing on a scene to be viewed, and an image intensifier tube. The image intensifier tube performs the image intensification process described above, and includes a photocathode liberating photo-electrons in response to light photons to convert the light energy received from the scene into electron patterns, a micro channel plate to multiply the electrons, a phosphor screen to convert the electron patterns into visible light, and possibly a fiber optic transfer window to invert the image. The control portion includes the electronic circuitry necessary for controlling and powering the image intensifier tube portion of the night vision system.
A factor limiting the performance of conventional image intensification tubes is the photocathode, and its spacing from the microchannel plate. That is, the photocathode of conventional image intensifier tubes is spaced sufficiently from the microchannel plate that a phenomenon known as halo occurs, and such that a higher than desired voltage must be maintained between the photocathode and the microchannel plate.
On the other hand, manufacturing economies, limitations, and practices have heretofore frustrated attempts to reduce the spacing dimension between a photocathode and the microchannel plate of an image intensifier tube. To place this problem in perspective, conventional spacing dimensions for GEN III image intensifier tubes are on the order of 250 .mu. meter (+ or - about 25 .mu. meter). This dimension is 0.000250 meter. Understandably, manufacturing tolerances and practices must be very precise to position a photocathode and microchannel plate at this distance from one another, parallel to one another--within tolerances, and without having these two structures touch one another. Further, the electric field which exists between these two structures is strongly affected by the spacing dimension between them. If the spacing is too small in conventional image intensifier tubes, then electrical discharge areas can occur--rendering the tube unusable. Similarly, too great of a spacing dimension results in a tube of sub-par performance.
A conventional photocathode for an infra-red type of sensor is known in accord with U.S. Pat. No. 3,959,045, issued May 25, 1976, to G. A. Antypas. The photocathode taught by the '045 patent is one version of the now-conventional Gen 3 photocathode described above.
However, the conventional spacing dimension used in conventional image intensifier tubes is much greater than desired. In order to allow the image intensifier tube to operate with a lower level of voltage applied between the photocathode and the microchannel plate, it is desirable to reduce the spacing between the photocathode and the microchannel plate, perhaps by as much as an order of magnitude below that spacing that is presently conventional. Such a reduction in spacing dimension between the photocathode and microchannel plate would, it is believed, also be effective to reduce or eliminate the halo phenomenon.